1. Field of the Invention
The present invention relates to a projection type display apparatus and more particularly to a liquid crystal projector which has an increased luminance and which is of a simplified structure produced with a reduced cost.
2. Description of the Related Art
FIG. 10 shows an optical system in a projection type display system constructed in accordance with the prior art. The optical system comprises a source of white light 1; a parabolic mirror 2 for converting the rays emitted from the light source 1 into parallel rays; dichroic mirrors 11a, 11b, 14a and 14b each for reflecting rays ranged in a specific region of wavelength; reflective mirrors 12 and 13; transmission type liquid crystal (display elements 17a, 17b and 17c each including two polarizing plates 15a, 15b and a liquid crystal cell 16 sandwiched between the polarizing plates; and a projecting lens 19 for enlarging and projecting images from the respective liquid crystal display elements 17 onto a screen 20.
On operation, rays emitted from the source of white light 1 are converted into parallel rays by the parabolic mirror 2, the parallel rays entering two dichroic mirrors 11a and 11b. At these dichroic mirrors 11a and 11b, the parallel white rays are separated into rays 7a, 7b and 7c of three colors R, G and B. These rays 7a, 7b and 7c illuminate the liquid crystal display elements 17a, 17b and 17c, respectively. In FIG. 10, each of the liquid crystal display elements 17 has a rectangular imaging face which is of a ratio of vertical side to horizontal side or aspect ratio equal to 3:4 for NTSC system or 9:16 for HDTV (High Definition TV) system. Images of colors R, G and B are synthesized at the dichroic mirrors 14a and 14b. The synthesized image is enlarged and projected by the projecting lens 19 onto the screen 20.
In such an arrangement of the prior art, the light emitted from the light source 1 is the natural light which is randomly variable in direction of output wavefront. Thus, polarizing components of the emitted beam 7 in a direction perpendicular to the direction of output wavefront in the entrance-side polarizing plate 15a will be absorbed by this polarizing plate 15a in the liquid crystal display element 17. Such an absorption results in loss in the projector. Theoretically, half of the light rays are lost at the entrance-side polarizing plate 15a. This is an obstacle to provision of a high-luminance projected image. The power of light absorbed by the polarizing plate 15a may increase it in temperature, leading to degradation of the selectivity of polarization.
The present invention is directed to overcome the aforementioned problems in the prior art.
It is therefore an object of the present invention to provide a projection type display apparatus which can realize a liquid crystal projector for projecting high-luminance images.